Commit 7e5b3c26 authored by Mark Gross's avatar Mark Gross Committed by Thomas Gleixner
Browse files

x86/speculation: Add Special Register Buffer Data Sampling (SRBDS) mitigation 



SRBDS is an MDS-like speculative side channel that can leak bits from the
random number generator (RNG) across cores and threads. New microcode
serializes the processor access during the execution of RDRAND and
RDSEED. This ensures that the shared buffer is overwritten before it is
released for reuse.

While it is present on all affected CPU models, the microcode mitigation
is not needed on models that enumerate ARCH_CAPABILITIES[MDS_NO] in the
cases where TSX is not supported or has been disabled with TSX_CTRL.

The mitigation is activated by default on affected processors and it
increases latency for RDRAND and RDSEED instructions. Among other
effects this will reduce throughput from /dev/urandom.

* Enable administrator to configure the mitigation off when desired using
  either mitigations=off or srbds=off.

* Export vulnerability status via sysfs

* Rename file-scoped macros to apply for non-whitelist table initializations.

 [ bp: Massage,
   - s/VULNBL_INTEL_STEPPING/VULNBL_INTEL_STEPPINGS/g,
   - do not read arch cap MSR a second time in tsx_fused_off() - just pass it in,
   - flip check in cpu_set_bug_bits() to save an indentation level,
   - reflow comments.
   jpoimboe: s/Mitigated/Mitigation/ in user-visible strings
   tglx: Dropped the fused off magic for now
 ]

Signed-off-by: default avatarMark Gross <mgross@linux.intel.com>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarTony Luck <tony.luck@intel.com>
Reviewed-by: default avatarPawan Gupta <pawan.kumar.gupta@linux.intel.com>
Reviewed-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Tested-by: default avatarNeelima Krishnan <neelima.krishnan@intel.com>
parent 93920f61

Documentation/ABI/testing/sysfs-devices-system-cpu

+1 −0
Original line number Diff line number Diff line
@@ -492,6 +492,7 @@ What: /sys/devices/system/cpu/vulnerabilities 
		/sys/devices/system/cpu/vulnerabilities/spec_store_bypass

		/sys/devices/system/cpu/vulnerabilities/l1tf

		/sys/devices/system/cpu/vulnerabilities/mds

		/sys/devices/system/cpu/vulnerabilities/srbds

		/sys/devices/system/cpu/vulnerabilities/tsx_async_abort

		/sys/devices/system/cpu/vulnerabilities/itlb_multihit

Date:		January 2018

Documentation/admin-guide/kernel-parameters.txt

+20 −0
Original line number Diff line number Diff line
@@ -4757,6 +4757,26 @@ 
			the kernel will oops in either "warn" or "fatal"

			mode.



	srbds=		[X86,INTEL]

			Control the Special Register Buffer Data Sampling

			(SRBDS) mitigation.



			Certain CPUs are vulnerable to an MDS-like

			exploit which can leak bits from the random

			number generator.



			By default, this issue is mitigated by

			microcode.  However, the microcode fix can cause

			the RDRAND and RDSEED instructions to become

			much slower.  Among other effects, this will

			result in reduced throughput from /dev/urandom.



			The microcode mitigation can be disabled with

			the following option:



			off:    Disable mitigation and remove

				performance impact to RDRAND and RDSEED



	srcutree.counter_wrap_check [KNL]

			Specifies how frequently to check for

			grace-period sequence counter wrap for the

arch/x86/include/asm/cpufeatures.h

+2 −0
Original line number Diff line number Diff line
@@ -362,6 +362,7 @@ 
#define X86_FEATURE_AVX512_4FMAPS	(18*32+ 3) /* AVX-512 Multiply Accumulation Single precision */

#define X86_FEATURE_FSRM		(18*32+ 4) /* Fast Short Rep Mov */

#define X86_FEATURE_AVX512_VP2INTERSECT (18*32+ 8) /* AVX-512 Intersect for D/Q */

#define X86_FEATURE_SRBDS_CTRL		(18*32+ 9) /* "" SRBDS mitigation MSR available */

#define X86_FEATURE_MD_CLEAR		(18*32+10) /* VERW clears CPU buffers */

#define X86_FEATURE_TSX_FORCE_ABORT	(18*32+13) /* "" TSX_FORCE_ABORT */

#define X86_FEATURE_PCONFIG		(18*32+18) /* Intel PCONFIG */
@@ -407,5 +408,6 @@ 
#define X86_BUG_SWAPGS			X86_BUG(21) /* CPU is affected by speculation through SWAPGS */

#define X86_BUG_TAA			X86_BUG(22) /* CPU is affected by TSX Async Abort(TAA) */

#define X86_BUG_ITLB_MULTIHIT		X86_BUG(23) /* CPU may incur MCE during certain page attribute changes */

#define X86_BUG_SRBDS			X86_BUG(24) /* CPU may leak RNG bits if not mitigated */



#endif /* _ASM_X86_CPUFEATURES_H */

arch/x86/include/asm/msr-index.h

+4 −0
Original line number Diff line number Diff line
@@ -128,6 +128,10 @@ 
#define TSX_CTRL_RTM_DISABLE		BIT(0)	/* Disable RTM feature */

#define TSX_CTRL_CPUID_CLEAR		BIT(1)	/* Disable TSX enumeration */



/* SRBDS support */

#define MSR_IA32_MCU_OPT_CTRL		0x00000123

#define RNGDS_MITG_DIS			BIT(0)



#define MSR_IA32_SYSENTER_CS		0x00000174

#define MSR_IA32_SYSENTER_ESP		0x00000175

#define MSR_IA32_SYSENTER_EIP		0x00000176

arch/x86/kernel/cpu/bugs.c

+106 −0
Original line number Diff line number Diff line
@@ -41,6 +41,7 @@ static void __init l1tf_select_mitigation(void); 
static void __init mds_select_mitigation(void);

static void __init mds_print_mitigation(void);

static void __init taa_select_mitigation(void);

static void __init srbds_select_mitigation(void);



/* The base value of the SPEC_CTRL MSR that always has to be preserved. */

u64 x86_spec_ctrl_base;
@@ -108,6 +109,7 @@ void __init check_bugs(void) 
	l1tf_select_mitigation();

	mds_select_mitigation();

	taa_select_mitigation();

	srbds_select_mitigation();



	/*

	 * As MDS and TAA mitigations are inter-related, print MDS
@@ -397,6 +399,97 @@ static int __init tsx_async_abort_parse_cmdline(char *str) 
}

early_param("tsx_async_abort", tsx_async_abort_parse_cmdline);



#undef pr_fmt

#define pr_fmt(fmt)	"SRBDS: " fmt



enum srbds_mitigations {

	SRBDS_MITIGATION_OFF,

	SRBDS_MITIGATION_UCODE_NEEDED,

	SRBDS_MITIGATION_FULL,

	SRBDS_MITIGATION_TSX_OFF,

	SRBDS_MITIGATION_HYPERVISOR,

};



static enum srbds_mitigations srbds_mitigation __ro_after_init = SRBDS_MITIGATION_FULL;



static const char * const srbds_strings[] = {

	[SRBDS_MITIGATION_OFF]		= "Vulnerable",

	[SRBDS_MITIGATION_UCODE_NEEDED]	= "Vulnerable: No microcode",

	[SRBDS_MITIGATION_FULL]		= "Mitigation: Microcode",

	[SRBDS_MITIGATION_TSX_OFF]	= "Mitigation: TSX disabled",

	[SRBDS_MITIGATION_HYPERVISOR]	= "Unknown: Dependent on hypervisor status",

};



static bool srbds_off;



void update_srbds_msr(void)

{

	u64 mcu_ctrl;



	if (!boot_cpu_has_bug(X86_BUG_SRBDS))

		return;



	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))

		return;



	if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED)

		return;



	rdmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_ctrl);



	switch (srbds_mitigation) {

	case SRBDS_MITIGATION_OFF:

	case SRBDS_MITIGATION_TSX_OFF:

		mcu_ctrl |= RNGDS_MITG_DIS;

		break;

	case SRBDS_MITIGATION_FULL:

		mcu_ctrl &= ~RNGDS_MITG_DIS;

		break;

	default:

		break;

	}



	wrmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_ctrl);

}



static void __init srbds_select_mitigation(void)

{

	u64 ia32_cap;



	if (!boot_cpu_has_bug(X86_BUG_SRBDS))

		return;



	/*

	 * Check to see if this is one of the MDS_NO systems supporting

	 * TSX that are only exposed to SRBDS when TSX is enabled.

	 */

	ia32_cap = x86_read_arch_cap_msr();

	if ((ia32_cap & ARCH_CAP_MDS_NO) && !boot_cpu_has(X86_FEATURE_RTM))

		srbds_mitigation = SRBDS_MITIGATION_TSX_OFF;

	else if (boot_cpu_has(X86_FEATURE_HYPERVISOR))

		srbds_mitigation = SRBDS_MITIGATION_HYPERVISOR;

	else if (!boot_cpu_has(X86_FEATURE_SRBDS_CTRL))

		srbds_mitigation = SRBDS_MITIGATION_UCODE_NEEDED;

	else if (cpu_mitigations_off() || srbds_off)

		srbds_mitigation = SRBDS_MITIGATION_OFF;



	update_srbds_msr();

	pr_info("%s\n", srbds_strings[srbds_mitigation]);

}



static int __init srbds_parse_cmdline(char *str)

{

	if (!str)

		return -EINVAL;



	if (!boot_cpu_has_bug(X86_BUG_SRBDS))

		return 0;



	srbds_off = !strcmp(str, "off");

	return 0;

}

early_param("srbds", srbds_parse_cmdline);



#undef pr_fmt

#define pr_fmt(fmt)     "Spectre V1 : " fmt
@@ -1528,6 +1621,11 @@ static char *ibpb_state(void) 
	return "";

}



static ssize_t srbds_show_state(char *buf)

{

	return sprintf(buf, "%s\n", srbds_strings[srbds_mitigation]);

}



static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr,

			       char *buf, unsigned int bug)

{
@@ -1572,6 +1670,9 @@ static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr 
	case X86_BUG_ITLB_MULTIHIT:

		return itlb_multihit_show_state(buf);



	case X86_BUG_SRBDS:

		return srbds_show_state(buf);



	default:

		break;

	}
@@ -1618,4 +1719,9 @@ ssize_t cpu_show_itlb_multihit(struct device *dev, struct device_attribute *attr 
{

	return cpu_show_common(dev, attr, buf, X86_BUG_ITLB_MULTIHIT);

}



ssize_t cpu_show_srbds(struct device *dev, struct device_attribute *attr, char *buf)

{

	return cpu_show_common(dev, attr, buf, X86_BUG_SRBDS);

}

#endif

CRA Git | Maintained and supported by SUSTech CRA and CCSE